The horse’s small intestine is composed of the duodenum, the jejunum, and the ileum. It is almost 70 feet long and has a capacity of about 15 gallons. Transit of ingested material through the small intestine takes anywhere from 30 to 120 minutes. When food passes from the stomach, the small intestine receives a continuous flow of pancreatic juices used to break down the food particles enzymatically to a small enough size for absorption into the bloodstream.

This flow is controlled by nervous or hormonal factors associated with a meal. The small intestine is, or should be if the feeding program is properly designed, the major zone of absorption for simple sugars derived from starch digestion, of amino acids from protein digestion, of free fatty acids resulting from digestion of fat in the diet, of the fat-soluble vitamins A, D and E, and of some minerals.

Parasites in the intestines can severely compromise the horse’s ability to get adequate nutrition from the food he is eating. Perhaps not as easily detected as parasite infestation are other problems in the small intestine that could interfere with a healthy digestive tract: food may be inadequately prepared as it enters the small intestine, or the quantity of the nutrients normally digested there may overwhelm the capacity of the small intestine. The major problem is the passage of these nutrients into the large intestine resulting in the loss of potential nutrients for the horse and causing other changes in the large intestine that may lead to colic or laminitis.

The first step in keeping the small intestine healthy is to initiate a proper deworming program. Second, offer feed ingredients that are highly digestible. Treatment of grains can increase digestibility. Oats, either whole or crimped, are fairly well digested by the horse. On the other hand, corn and barley are not very well digested in the small intestine. The smaller the particle size for these grains, the more digestible; for example, ground corn is more digestible than whole corn. Further treatment with heat such as steam-flaking or micronizing will significantly improve the digestibility of these grains.

Fat added to the diet should be digested in the small intestine. Different fat sources have different digestibilities. For example, corn oil has been found to be up to 95% digestible, while the digestibility of animal fat is only about 75%. If too large a quantity of fat is fed, then enough may pass into the large intestine to upset the microbial population and interfere with nutrient absorption.